Process and apparatus for producing inorganic fullerene-like nanoparticles

ABSTRACT

The present invention provides a process for obtaining fullerene-like metal chalcogenide nanoparticles, comprising feeding a metal precursor (INi) selected from metal halide, metal carbonyl, organo-metallic compound and metal oxyhalide vapor into a reaction chamber ( 12 ) towards a reaction zone to interact with a flow of at least one chalcogen material (IN 2 ) in gas phase, the temperature conditions in said reaction zone being such to enable the formation of the fullerene-like metal chalcogenide nanoparticles product. The present invention further provides novel IF metal chalcogenides nanoparticles with spherical shape and optionally having a very small or no hollow core and also exhibiting excellent tribological behavior. The present invention further provides an apparatus for preparing various IF nanostructures.

FIELD OF THE INVENTION

This invention relates to a chemical process and apparatus for producinginorganic fullerene-like nanoparticles.

LIST OF REFERENCES

The following references are considered to be pertinent for the purposeof understanding the background of the present invention:

-   -   1. L. Rapoport, Yu. Bilik, Y. Feldman, M. Homyonfer, S. Cohen        and R. Tenne, Nature, 1997, 387, 791;    -   2. C. Schffenhauer, R. Popovitz-Biro, and R. Tenne, J. Mater.        Chem. 2002, 12, 1587-1591    -   3. Jun. Chen, Suo-Long Li, Zhan-Liang Tao and Feng Gao, Chem.        Commun. 2003, 980-981;    -   4. WO 97/44278;    -   5. Y. Feldman, V. Lyalkhovitskaya and R. Tenne, J. Am. Chem.        Soc. 1998, 120, 4176;    -   6. A. Zak, Y. Feldman, V. Alperovich, R. Rosentsveig and R.        Tenne, J. Am. Chem. Soc. 2000, 122, 11108;    -   7. Y. Feldman, A. Zak, R. Popovitz-Biro, R. Tenne, Solid State        Sci. 2000, 2, 663;    -   8. WO 01/66462;    -   9. WO 02/34959;    -   10. Xiao-Lin Li, Jian-Ping Ge and Ya-Dong Li, Chem. Eur. J.        2004, 10, 6163-6171; and    -   11. T. Tsirlina and V. Lyaldiovitskaya, S. Fiechter, and R.        Tenne, J. Mater. Res. 2000, 15, 2636-2646.

BACKGROUND OF THE INVENTION

Carbon nanoparticles having a layered configuration are known asfullerene nanoparticles. Generally, there are three main types offullerene-related carbon particles: fullerenes (C₆₀, C₇₀, etc.);nested-fullerene nanoparticles (in the form of onions), and nanotubes.Analogous fullerene-like nanoparticles can be obtained from a number ofinorganic materials with layered structure, and are known as inorganicfullerene-like materials.

Inorganic fullerene-like (abbreviated hereinafter “IF”) nanoparticlesand nanotubes are attractive due to their unique crystallographicmorphology and their interesting physical properties.

Layered transition-metal dichalcogenides MS₂ (such as WS₂ and MoS₂) areof great interest as they act as host lattices by reacting with avariety of guest atoms or molecules to yield intercalation compounds, inwhich the guest is inserted between the host layers. Accordingly, IFtransition metal dichalcogenides may be used for instance, for hydrogenstorage.

Furthermore, disulfides of molybdenum and tungsten belong to a class ofsolid lubricants useful in vacuum, space and other applications whereliquids are impractical to use. IF nanoparticles can be used asadditives to various kinds of oils and greases to enhance theirtribological behavior¹. Furthermore, different coatings with impregnatedIF nanoparticles were shown to exhibit self-lubricating behavior.

IF nanoparticles may also be used for other possible applications suchas battery cathodes, catalysis, nanoelectronic and magnetic informationstorage.

The first closed-cage fullerene-like nanoparticles and nanotubes of WS₂were obtained via sulfidization of thin films of the respectivetrioxides in 1992, followed by MoS₂ and the respective diselenides.

Numerous IF nanostructures have been synthesized using differentmetodologies. The first report related to IF-MS₂ (IF-NbS₂) structuresobtained by the reaction of the metal chloride (NbCl₅) and H₂S². Lateron, Jun Chen et al.³ used a low-temperature gas reaction to synthesizeTiS₂ nanotubes. The reaction involved heating TiCl₄, H₂, and H₂S insidea horizontal furnace at a relatively low temperature of 450° C. and inthe absence of oxygen and water.

Another method and apparatus for preparing inorganic fullerene-likenanoparticles of a metal, e.g. transition metal chalcogenide having adesired size and shape in high yields and macroscopic quantities, isdescribed in WO 97/44278⁴. This method utilizes (a) dispersing solidparticles of at least one non-volatile metal oxide material having thepreselected size and shape; and (b) heating the solid particles of thenon-volatile metal material in a reducing gaseous atmosphere containingat least one chalcogen material for a time and a temperature sufficientto allow the metal material precursor and the chalcogen material toreact and form at least one layer of metal chalcogenide, the at leastone layer of metal chalcogenide encaging the surface of the solidparticles to form the fullerene-like particles.

The synthesis of IF-WS₂ involves a solid-gas reaction, where thenanocrystalline tungsten oxide, serving as a precursor, reacts with H₂Sgas at elevated temperatures⁵. In a different procedure, IF-MoS₂nanoparticles are prepared in the gas phase, upon in-situ reduction andcondensation of the MoO₃ vapor and subsequent sulfidization by H₂S⁶.

The availability of fullerene-like MoS₂ and WS₂ nanoparticles in largeamounts paved the way for a systematic investigation of theirproperties. Both IF-WS₂ and IF-MoS₂ nanoparticles were found to providebeneficial tribological behavior under harsh conditions¹, suggestingextensive number of tribological applications for these nanoparticles,eliciting substantial industrial interest.

Mass production of IF-WS₂ was enabled by the construction of first afalling bed and subsequently fluidized bed reactors⁷.

Reactors for mass production of IF-WS₂ and IF-MoS₂ are described in WO01/66462 and WO 02/34959, respectively^(8,9).

The reported IF-WS₂ and IF-MoS₂ ⁵⁻⁷ were synthesized from theircorresponding oxide crystallite that served as a template for the growthof the sulfide nanoparticles. The growth of the sulfide layers in eachparticle starts on the top surface of the partially reduced oxidenanoparticle terminating in its core. This diffusion-controlled reactionis rather slow, lasting a few hours. The final nanoparticles consist ofdozens of sulfide layers and a hollow core occupying 5-10% of the totalvolume of the nanoparticles.

In another research, large-scale MoS₂ and WS₂ IF nanostructures(onion-like nanoparticles and nanotubes) and three-dimensionalnanoflowers were selectively prepared through an atmospheric pressurechemical vapor deposition process from metal chlorides (e.g. MoCl₅ andWCl₆) and sulfur¹⁰. In this technique, selectivity was achieved byvarying the reaction temperature, with 750° C. favoring the nanotubesand 850° C. the fullerene-like nanoparticles.

In a further research, tungsten diselenide closed-cage nanoparticleswere synthesized by the reaction of prevaporized Se with WO₃ powder in areducing atmosphere¹¹. The selenium vapor was brought to the mainreaction chamber by a carrier gas. The growth mechanism of the IF-WSe₂nanoparticles was outside-in. This growth mode is analogous to thepreviously reported growth of IF-WS₂ using the reaction between WO₃nanoparticles and H₂S gas⁵.

SUMMARY OF THE INVENTION

There is a need in the art to facilitate production of inorganicfullerene-like particles by providing a novel process and apparatus withimproved capability to control the shape and size of the structure beingproduced. Also, there is a need in the art to produce nanoparticleshaving spherical shape, thus having improved properties, such astribological, optical, etc.

It was found by the inventors that the known mechanisms for thesynthesis of IF-WS₂ from metal trioxide powder and the synthesis ofIF-MoS₂ from the evaporated metal trioxide, are not suitable for othermetals such as titanium. For instance, the titanium dioxide can not beeasily sulfidized even at the relatively high temperature of up to 1450°C. Also, although the sulfidization of tungsten or molybdenum dioxideresults in respective disulfide, the desired morphology of the particleis not obtained.

Furthermore, the inventors have found a more rapid way for making thesynthesis of IF nanoparticles that yields a desired spherical shape anda relatively narrow size distribution of produced nanoparticles. The IFnanoparticles synthesized by the technique of the present invention havesmaller hollow core (substantially not exceeding 5-10 nm) and theycontain many more layers (typically, 50-120 layers) as compared to thosesynthesized from the metal oxides, which have a relatively large hollowcore (more than 20 nm) and fewer number of layers (20-40). Therefore,the presently synthesized IF nanoparticles are expected to revealimproved tribological behavior, which is confirmed by preliminarymeasurements.

Thus, the present invention provides a process for producing inorganicfullerene-like (IF) nanoparticles having well defined size and shape,from commercially available reactants and in a rather fast reaction. Thelarge number of molecular layers, i.e. 50-120 in the present synthesisis advantageous for tribological applications where the lifetime of theNan particle is determined by the gradual deformation and peeling-off ofthe outer layers of the nanoparticle.

The process of the present invention occurs in the gas phase, and issuitable for mass production of inorganic fullerene-like nanoparticlesof metal chalcogenides. The process is based on a reaction between ametal precursor, e.g. metal halide, metal oxyhalide, metal carbonyl ororgano-metallic compound (hereinafter termed “metal containingprecursor” or “metal precursor”) and a reacting agent, e.g. chalcogenmaterial, both in the gas phase. The use of metal carbonyls, forexample, has the advantage that its decomposition in the reactor leadsto the release of CO which is a strongly reducing agent and allows toovercome the sensitivity of this reaction to oxidizing atmosphere.

Thus according to a first aspect thereof, the present invention providesa process for producing inorganic fullerene-like (IF) metal chalcogenidenanoparticles, the process comprising:

(a) feeding a metal precursor selected from metal halide, metalcarbonyl, organo-metallic compound and metal oxyhalide vapor into areaction chamber towards a reaction zone to interact with a flow of atleast one chalcogen material in gas phase, the temperature conditions insaid reaction zone being such as to enable the formation of theinorganic fullerene-like (IF) metal chalcogenide nanoparticles.

According to a preferred embodiment, the process comprises:

(b) controllably varying the flow of said metal precursor into saidreaction chamber to control the amount, shape and size of theso-produced IF fullerene-like metal chalcogenide nanoparticles in solidform.

Preferably, the vapor of the metal precursor is fed into the reactionchamber to flow towards the reaction zone along a vertical path, e.g.along an upward/downward direction that is opposite with respect to thatof the chalcogen material that is being fed in a downward/upwarddirection.

The nanoparticles produced by the process of the invention arecharacterized by narrow size distribution and large number of molecularlayers.

The invention also provides IF metal chalcogenide nanoparticles having aplurality of molecular layers and characterized in that the number ofsaid molecular layers exceeds 40, preferably exceeds 50 and at timesexceeds 60 and even 70 layers. According to one embodiment of theinvention there is provided a product comprising a plurality of IF metalchalcogenide nanoparticles, a substantial portion of which having anumber of molecular layers exceeding 40, preferably exceeds 50 and attimes exceeds 60 and even 70 layers. The substantial portion istypically more than 40% out of the nanoparticles, preferably more than50%, 60%, 70%, 80% and at times even more than 90% out of the totalnumber of the IF nanoparticles.

Furthermore, the IF fullerene-like metal chalcogenide nanoparticlesproduced by the process of the present invention optionally have nohollow core or a very small hollow core (not exceeding 5-10 nm).

The term “very small hollow core” as used herein means that thenanoparticles produced by the process of the present invention have ahollow core which is not exceeding 5 nm or occupying no more than 0-5%of the total volume of the nanoparticles.

The term “nanoparticles” as used herein refers to multi-layered,spherical, or close to spherical particle having a diameter in the rangefrom about 10 nm to about 300 nm, preferably from about 30 nm to about200 nm. The nanoparticles of the invention may typically have 50-120concentric molecular layers.

The nanoparticles obtained by the process of the present invention havea spherical or close to spherical shape and optionally have no hollowcore. The provision of a very small hollow core or even absence of suchcore may be explained by the mechanism of growth of the nanoparticles,namely from the central portion (nucleai of product) towards theperipheral portion, rather than the opposite direction carried out inthe known processes.

Preferably, the term “mental” as used herein refers to In, Ga, Sn or atransition metal.

A transition metal includes all the metals in the periodic table fromtitanium to copper, from zirconium to silver and from hafnium to gold.Preferably, the transition metals are selected from Mo, W, V, Zr, Hf,Pt, Pd, Re, Nb, Ta, Ti, Cr and Ru.

A chalcogen used in the invention is S, Se or Te, and the chalcogenmaterial is selected from a chalcogen, a compound containing achalcogen, a mixture of chalcogens, a mixture of compounds containing achalcogen, and a mixture of a chalcogen and a compound containing achalcogen.

The chalcogen material is preferably a chalcogen compound containinghydrogen, more preferably H₂S, H₂Se and/or H₂Te. Alternatively, insteadof H₂X (X=S, Se, Te) it is possible to use elemental chalcogen under theflow of hydrogen with H₂X being formed in-situ during the reaction time.The chalcogen material may optionally be mixed with a reducing agentsuch as hydrogen and/or Co.

In a preferred embodiment of the invention, an inert carrier gas is usedto drive a flow of the chalcogen material and a flow of the vaporizedmetal precursor into the reaction chamber. Non limiting examples ofinert gases that may be used in the process of the present invention areN₂, He, Ne, Ar, Kr and Xe.

The term “precursor” as used herein means any suitable starting materialor materials. The precursor in the process of the present invention maybe any metal containing compound that can be vaporized without or withits decomposition. Suitable metal containing precursors that may be usedin the process of the present invention are, for example, metal halides,metal carbonyls, organo-metallic compounds and metal oxyhalides. Morespecific examples of metal containing precursors that may be used in theprocess of the present invention are TiCl₄, WCl₆, WCl₅, WCl₄, WBr₅,WO₂Cl₂, WOCl₄, MoCl₅, Mo(CO)₅ and W(CO)₆, Ga(CH₃)₃, W(CH₂CH₃)₅, In(CH₃)₃and the like.

A list of metal precursor compounds that can be used in the process ofthe present invention is given in Table 1 below.

TABLE 1 Examples of metal precursors Name Formula mp, ° C. bp, ° C.Chromium carbonyl Cr(CO)₆ 130 (dec) subl Chromium (III) iodide CrI₃ 500(dec) Chromium (IV) chloride 600 (dec) Chromium (IV) fluoride CrF₄ 277Chromium (V) fluoride CrF₅ 34 117 Chromium (VI) fluoride CrF₆ 100 (dec)Cromyl chloride CrO₂Cl₂ −96.5 117 Trimethylgallium Ga(CH₃)₃ −15.8 55.7Hafnium bromide HfBr₄ 424 (tp)  323 (sp)  Hafnium chloride HfCl₄ 432(tp)  317 (sp)  Hafnium iodide HfI₄ 449 (tp)  394 (sp)  TrimethylindiumIn(CH₃)₃ 88 133.8 Molybdenum carbonyl Mo(CO)₆ 150 (dec) subl Molybdenum(V) chloride MoCl₅ 194 268 Molybdenum (V) fluoride MoF₅ 67 213Molybdenum (V) MoOCl₃ 297 subl oxytrichloride Molybdenum (VI) fluorideMoF₆ 17.5 34 Molybdenum (VI) MoOF₄ 98 oxytetrafluoride Molybdenum (VI)MoOCl₄ 101 oxytetrachloride Molybdenum (VI) MoO₂Cl₂ 175 dioxydichlorideNiobium (IV) chloride NbCl₄ Niobium (IV) fluoride NbF₄ 350 (dec) Niobium(IV) iodide NbI₄ 503 Niobium (V) bromide NbBr₅ 254 360 Niobium (V)chloride NbCl₅ 204.7 254 Niobium (V) fluoride NbF₅ 80 229 Niobium (V)iodide NbI₅ 200 (dec) Niobium (V) oxybromide NbOBr₃ 320 (dec) sublNiobium (V) oxychloride NbOCl₃ subl Niobium (V) dioxyfluoride NbO₂FPalladium (II) bromide PdBr₂ 250 (dec) Palladium (II) iodide PdI₂ 360(dec) Platinum (II) bromide PtBr₂ 250 (dec) Platinum (II) chloride PtCl₂581 (dec) Platinum (II) iodide PtI₂ 325 (dec) Platinum (III) bromidePtBr₃ 200 (dec) Platinum (III) chloride PtCl₃ 435 (dec) Platinum (IV)bromide PtBr₄ 180 (dec) Platinum (IV) chloride PtCl₄ 327 (dec) Platinum(IV) fluoride PtF₄ 600 Platinum (IV) iodide PtI₄ 130 (dec) Platinum (VI)fluoride PtF₆ 61.3 69.1 Rhenium carbonyl Re₂(CO)₁₀ 170 (dec) Rhenium(III) bromide ReBr₃ 500 (subl) Rhenium (III) chloride ReCl₃ 500 (dec)Rhenium (III) iodide ReI₃ (dec) Rhenium (IV) chloride ReCl₄ 300 (dec)Rhenium (IV) fluoride ReF₄ 300 (subl) Rhenium (V) bromide ReBr₅ 110(dec) Rhenium (V) chloride ReCl₅ 220 Rhenium (V) fluoride ReF₅ 48 220Rhenium (VI) chloride ReCl₆ 29 Rhenium (VI) fluoride ReF₆ 18.5 33.7Rhenium (VI) ReOCl₄ 29.3 223 oxytetrachloride Rhenium (VI) ReOF₄ 108 171oxytetrafluoride Rhenium (VII) fluoride ReF₇ 48.3 73.7 Rhenium (VII)trioxycloride ReO₃Cl 4.5 128 Rhenium (VII) ReO₃F 147 164 trioxyfluorideRhenium (VII) ReO₂F₃ 90 185 dioxytrifluoride Rhenium (VII) ReOF₅ 43.8 73oxypentafluoride Ruthenium dodecacarbonyl Ru₃(CO)₁₂ 150 (dec) Ruthenium(III) bromide RuBr₃ 400 (dec) Ruthenium (III) chloride RuCl₃ 500 (dec)Ruthenium (III) fluoride RuF₃ 600 (dec) Ruthenium (III) iodide RuI₃Ruthenium (IV) fluoride RuF₄ 86.5 227 Ruthenium (V) fluoride RuF₅ 54Tantalum (V) bromide TaBr₅ 265 349 Tantalum (V) chloride TaCl₅ 216239.35 Tantalum (V) fluoride TaF₅ 95.1 229.2 Tantalum (V) iodide TaI₅496 543 Titanium (III) bromide TiBr₃ Titanium (III) chloride TiCl₃ 425(dec) Titanium (IV) bromide TiBr₄ 39 230 Titanium (IV) chloride TiCl₄−25 136.45 Titanium (IV) fluoride TiF₄ 284 subl Titanium (IV) iodideTiI₄ 150 377 Tungsten carbonyl W(CO)₆ 170 (dec) subl Tungsten (II)bromide WBr₂ 400 (dec) Tungsten (II) chloride WCl₂ 500 (dec) Tungsten(II) iodide WI₂ Tungsten (III) bromide WBr₃  80 (dec) Tungsten (III)chloride WCl₃ 550 (dec) Tungsten (V) bromide WBr₅ 286 333 Tungsten (V)chloride WCl₅ 242 286 Tungsten (V) fluoride WF₅  80 (dec) Tungsten (V)oxytribromide WOBr₃ Tungsten (V) oxytrichloride WOCl₃ Tungsten (VI)bromide WBr₆ 309 Tungsten (VI) chloride WCl₆ 275 246.75 Tungsten (VI)WO₂Br₂ dioxydibromide Tungsten (VI) WO₂Cl₂ 265 dioxydichloride Tungsten(VI) WO₂I₂ dioxydiiodide Tungsten (VI) fluoride WF₆ 2.3 17 Tungsten (VI)WOBr₄ 277 327 oxytetrabromide Tungsten (VI) WOCl₄ 211 227.55oxytetrachloride Tungsten (VI) WOF₄ 106 186 oxytetrafluoride Vanadiumcarbonyl V(CO)₆  60 (dec) subl Vanadium (IV) chloride VCl₄ −25.7 148Vanadium (IV) fluoride VF₄ 325 (dec) subl Vanadium (V) fluoride VF₅ 19.548.3 Vanadyl bromide VOBr 480 (dec) Vanadyl chloride VOCl 700 (dec)Vanadyl dibromide VOBr₂ 180 (dec) Vanadyl dichloride VOCl₂ 380 (dec)Vanadyl difluoride VOF₂ Vanadyl tribromide VOBr₃ 180 (dec) Vanadyltrichloride VOCl₃ −79 127 Vanadyl trifluoride VOF₃ 300 480 Zirconiumchloride ZrCl₄ 437 (tp)  331 (sp)  Zirconium fluoride ZrF₄ 932 (tp)  912(sp)  Zirconium iodide ZrI₄ 499 (tp)  431 (sp)  Abbreviations:(dec)—decomposes (sp)—sublimation point (subl)—sublimes (tp)—triplepoint

According to a preferred embodiment of the invention, the processfurther comprises at least one, preferably both of the following steps:

(c) terminating the feeding of the metal precursor vapor into thereaction chamber by stopping heating of the metal precursor;

(d) cooling the reaction zone and collecting the obtained fullerene-likemetal chalcogenide nanoparticles.

In another preferred embodiment, the process may comprise driving a flowof an inert gas into the reaction zone after step (c) and before step(d).

In a further preferred embodiment, the process may farther compriseannealing to allow the precursor to react completely.

As indicated above, the temperature profile (conditions) used in thereaction zone is preferably such so as to enable the formation of thenanoparticles such that the nuclei of the nanoparticles have essentiallyno or very small hollow core. This results, among others, from the factthat formation of the nanoparticles is thorough a mechanism involvinggrowth of the nanoparticles from the central portion (nuclei of product)towards the peripheral portion.

Preferably, the temperature within the reaction zone is in the range of500° C. to 900° C., depending on the particular material beingsynthesized by the process (see examples below). The gradient of thetemperature within the reactor provides lowering of the temperaturetowards the filter.

In the process of the present invention, the amount, morphology and sizeof the nanoparticles are controlled by the flow of the metal precursorvapor. This flow may be controlled by adjusting the rate of the flow ofan inert gas driving the vapor into the reaction chamber; and/oradjusting the temperature used for heating the metal precursor to obtaina vapor thereof.

The heating temperature of the metal precursor is preferably very closeto its boiling point. More specifically, it is in the range of between50 degree below the boiling point and up to the boiling point of saidmetal precursor.

The process described above allows the preparation of nanoscaleinorganic fullerene-like (IF) metal chalcogenides having spherical shapeoptionally with a very small or no hollow core. The metal chalcogenidesare preferably selected from TiS₂, TiSe₂, TiTe₂, WS₂, WSe₂, WTe₂, MoS₂,MoSe₂, MoTe₂, SnS₂, SnSe₂, SnTe₂, RuS₂, RuSe₂, RuTe₂, GaS, GaSe, GaTe,In₂S₃, In₂Se₃, In₂Te₃, InS, InSe, Hf₂S, HfS₂, ZrS₂, VS₂, ReS₂ and NbS₂.

According to one preferred embodiment of the invention, novel TiS₂nanoparticles with fullerene-like structure having quite a perfectlyspherical shape and consisting of up to 120 concentric molecular layers,were obtained by the reaction of TiCl₄ and H₂S, using a verticalreactor. The obtained IF-TiS₂ exhibited excellent tribological behaviorresulting probably from their close to a spherical shape which promotesrolling friction.

An apparatus of the present invention includes a reaction chamber, and aseparate evaporation chamber, which is operated and whose connection tothe reaction chamber is controllably operated to control the shape, sizeand amount of the product being produced. The control of the outputparameters of the process (the shape, size and amount of thenanoparticles) is significantly improved by utilizing a verticalconfiguration of the reaction chamber. Thus, the present inventionprovides according to a further aspect thereof, an apparatus forpreparing IF nanostructures, the apparatus comprising: a reactionchamber having inlets for inputting reacting gases and an outlet; aseparate evaporation chamber for separately preparing a precursor vapor;and a control unit configured and operable for controlling the precursorvapor flow into the reaction chamber.

Preferably, the reaction chamber is a vertical chamber with the gasinlet accommodated so as to provide the reacting gases flow in oppositedirections towards a reaction zone where they meet and react with eachother. Preferably, the control unit comprises a bypass arrangementassociated with the evaporation chamber. This bypass is configured andoperable to provide a flow of clean inert gas instead of one enrichedwith vaporized precursor at certain moments of the reaction as describedfor instance, in Example 1 below. This improvement is of importance forthe synthetic procedure preventing the flow of the highly reactiveprecursor during the heating up and cooling down steps of the synthesis.

According to yet another broad aspect of the invention, there isprovided an apparatus for preparing IF nanostructures, the apparatuscomprising: (i) a reaction chamber configured to be vertically orientedduring the apparatus operation, and having gas inlets located at top andbottom sides of the chamber so as to direct a precursor vapor and theother reacting gas in opposite directions towards a reaction zone wherethe gases meet and react with each other; (ii) a separate evaporationchamber configured and operable for separately preparing the precursorvapor and feeding it to the respective inlet of the reaction chamber;and (c) a control unit configured and operable for controlling theprecursor vapor flow into the reaction chamber.

BRIEF DESCRIPTION OF THE DRAWINGS

In order to understand the invention and to see how it may be carriedout in practice, preferred embodiments will now be described, by way ofnon-limiting examples only, with reference to the accompanying drawings,in which:

FIG. 1 exemplifies a preferred configuration of an apparatus of thepresent invention utilizing a vertical reaction chamber associated witha separate evaporation chamber.

FIG. 2 is a schematic illustration of an apparatus utilizing ahorizontal reaction chamber.

FIG. 3 is the TEM image of IF-TiS₂ nanoparticle, produced in ahorizontal reactor.

FIG. 4 is the TEM image of a typical IF-TiS₂ nanoparticle, produced in avertical reactor. The interlayer distance is 5.8 Å and the diameter ofthe nanoparticle is larger than 70 nm.

Insert shows the Fast Fourier Transform (FFT) of the shown nanoparticle.

FIG. 5A is the HRTEM image of a part of an IF-TiS₂ nanoparticle producedin a vertical reactor with an overlay of the simulated TiS₂ pattern(view down [110], simulation with thickness 20 nm and defocus of −20nm).

FIG. 5B shows the measurement of the interlayer distance by HRTEM.

FIG. 6 shows the typical IF-WS₂ obtained from WO₂Cl₂ and H₂S in ahorizontal reactor.

FIG. 7A is the magnified TEM image of a group of IF-WS₂ nanoparticlesreceived in a reaction of WCL₄ and H₂S in a vertical reactor.

FIG. 7B is the TEM image of individual closed-caged IF-WS₂ nanoparticlereceived in a reaction of WCL₄ and H₂S in a vertical reactor.

FIG. 8 shows the WS₂ nanoparticle obtained from WCl₅ precursor in avertical reactor.

FIG. 9 is a TEM image of a small WS₂ nanoparticles obtained from WCl₆ ina vertical reactor

FIG. 10A is a TEM image of a group of MoS₂ nanoparticles obtained fromMoCl₅ in a vertical reactor.

FIG. 10B is a TEM image of small (20 nm) IF-MoS₂ obtained from MoCl₅ ina vertical reactor.

FIG. 11A shows IF-WS₂ synthesized from WO₃ by methods known in the art.

FIG. 11B shows IF-TiS₂ synthesized from TiCl₄. Each nanoparticle has adiameter ca. 60 nm.

Note the difference between FIGS. 11A and 11B in topology, number oflayers and the absence of a hollow core in IF-TiS₂.

DETAILED DESCRIPTION OF THE INVENTION

The principles of the process of the present invention will be explainedhereinbelow with reference to the preparation of closed-cage IFnanoparticles of TiS₂. However, it should be understood that thediscussion is not limited to that specific material but it applies to IFmetal chalcogenides in general.

IF nanoparticles of TiS₂ were synthesized through the reaction of TiCl₄and H₂S. The obtained nanoparticles have no or very small hollow coreand they consist of 80-100 molecular sheets with quite a perfectspherical shape. The IF nanoparticles were prepared by two reactorassemblies: a horizontal reactor and a vertical reactor.

Reference is made to FIG. 1 exemplifying a preferred configuration of anapparatus, generally designated 10, of the present invention suitable tobe used for synthesis of IF-nanoparticles with expected superiortribological behavior. The apparatus 10 includes a vertical reactionchamber 12 for mounting into an oven 15, a separate evaporation chamber14, and a control unit 16. An outlet 17 of the evaporation chamber 14 isconnectable to an inlet IN₁ of the reaction chamber 12 via a connectinggas-flow pipe (not shown here).

In the present example, the oven 15 is designed as a two-zone oven,operable to control the temperature profile in the reaction chamber. Thereaction chamber 12 has independent inlets IN₁ and IN₂ at opposite endsof the chamber 12 for inputting two reaction gases (agents),respectively, e.g., TiCl₄ and H₂S, and a gas outlet GO. Flows of thesereaction agents in opposite directions towards a reaction zone in thereaction chamber are assisted by inert gas, N₂, and a mixture of TiCl₄and H₂S gases is formed in the reacting zone. The control unit 16includes, inter alia, a mass flow controller 16A (e.g., TYLAN modelFC260 commercially available from Tylan General, USA) operable forcontrolling the flow-rate of H₂S, and a suitable flow controller 16B forcontrolling the flow of additional gas to dilute the H₂S by mixing itwith a stream of inert gas or inert gas mixed with a reducing agent.Further provided in the apparatus 10 is a filter 18 appropriatelyconfigured and accommodated to collect the product (nanoparticles). Thefilter 18 is preferably spatially separated from the inner walls of thereaction chamber 12.

The precursor (TiCl₄) vapors were prepared in advance in the separateevaporation chamber 14. The latter includes a gas-washing bottle 14A, atemperature source (not shown here) appropriately accommodated adjacentto the bottle 14A and operable to controllably heat the TiCl₄ liquidwhile in the bottle 14A. Valve arrangements 14B and 14C are provided topresent a bypass for the nitrogen flow. This bypass provides a flow ofclean nitrogen instead of one enriched with TiCl₄ at certain moments ofreaction. This improves the synthetic procedure since it prevents theflow of the highly reactive TiCl₄ precursor during the heating up andcooling down steps of the synthesis. To this end, each valve isshiftable (either by an operator or automatically) between its positionI (used for flushing the apparatus with pure nitrogen gas) and itsposition II (used for stopping the flush of the pure nitrogen gas)during the reaction. The precursor (TiCl₄) vapor was carried from theevaporation chamber 14 to the reaction chamber 12 by an auxiliary gasflow. The carrier gas is inert gas, which can be mixed with a reducingagent (H₂ or/and CO).

The control unit 16 is configured for controlling the gas flows and thetemperature sources' operation. The preheating temperature was found tobe a very significant factor, determining the amount of precursorsupplied to the reaction chamber 12. The flow-rate of nitrogen throughthe bottle 14A affects the stream of the titanium tetrachlorideprecursor as well.

This two-chamber design apparatus with the vertical configuration of thereaction chamber considerably improves the size and shape control of thesynthesized nanoparticles. The nucleation and growth mechanismestablished with the vertical reaction chamber (FIG. 1) providenanoparticles with quite a perfect spherical shape; small or no hollowcore and many layers, which are ideally suited for alleviating frictionand wear, as well as other different applications such as ultra strongnanocomposites, very selective and reactive catalysts, photovoltaicsolar cells, etc.

Using similar reactions, the nucleation and growth mechanism is likelyto provide many other kinds of IF nanoparticles with expected superiortribological behavior.

FIG. 2 shows another example of an apparatus, generally at 100. Theapparatus 100 includes a horizontal reaction chamber 112 associated witha single-zone oven 115, and a separate evaporation chamber 14 configuredas described above. The reaction chamber 112 has an inlet arrangement IN(for inputting reaction agents TiCl₄ and H₂S) and an outlet arrangementOA. A control unit 16 is used for controlling the operation of the oven115 to thereby control the temperature profile in the reaction chamber112. The flow-rate of H₂S, as well as that of N₂, is appropriatelycontrolled as described above. The TiCl₄ vapors were obtained bypreheating the liquid TiCl₄ in a gas-washing bottle (evaporationchamber). The TiCl₄ vapor is carried from the evaporation chamber 14 tothe reaction chamber 112 by an auxiliary N₂ gas flow. The resultingproduct (TiS₂ powder) is collected for analysis on the surface of thereaction chamber.

EXAMPLES Example 1 Preparation of IF-TiS₂ Nanoparticles in the VerticalReactor Based Apparatus of FIG. 1

In order to maintain a water and oxygen free atmosphere, the reactionchamber 12 was permanently maintained at 500° C. and a flow of N₂ gas(20 ml/min) until shortly before the run starts, when it is withdrawnfrom the oven 15. At this point, the reaction chamber 12 was opened andcleaned. At the beginning of the process, the reaction chamber 12 wasclosed hermetically from outside the oven, and the reaction gases,except for precursor (titanium tetrachloride), were supplied to theinlets flushing the system for 10-15 min. The slight overpressure (ca.1.1 bars) was maintained at a constant value by the gas trap in the exitGO of the gases from the reaction chamber 12. This procedure eliminatesmost of the residual atmospheric gases, like water vapor and oxygen fromthe reaction chamber. This step is very important for the synthesis,since both the final product (TiS₂) and especially the precursor (TiCl₄)are very sensitive to moisture. Subsequently, the reactor was insertedinto the oven 15.

Independent inlets IN₁ and IN₂ for both reaction gases i.e. TiCl₄ andH₂S were used, with the mixture of the reagents being formed in thereaction chamber itself. The flow-rate of H₂S (3-20 cc/min) wascontrolled by means of a TYLAN model FC260 mass flow-controller 16A. TheH₂S was diluted by mixing this gas with a stream of N₂ gas (10-200cc/min in this reaction) using another flow-controller 16B. The TiCl₄vapors were obtained by preheating the liquid TiCl₄ in the gas-washingbottle 14A of the evaporation chamber 14. The TiCl₄ vapor was carried tothe reaction chamber 12 by an auxiliary N₂ gas flow. The temperature ofthe TiCl₄ source was kept usually between 100 and 130° C., which isclose to its boiling point of 136.5° C. As indicated above, thepreheating temperature is a significant factor, determining the amountof precursor supplied to the reaction zone. The flow-rate of nitrogenthrough the bottle 14A (10-100 cc/min) affects the stream of thetitanium tetrachloride precursor as well. A small overpressure (1.1 bar)was maintained by using a gas trap filled with NaOH (5%) solution in thegas outlet of the reactor.

The temperature in the reaction chamber zone, where the two gases (TiCl₄and H₂S) mix and react, and near the filter 18 was usually variedbetween 650-750° C. This temperature was chosen based on the propertiesof the Ti-S system.

Several experiments have been run at higher temperatures (up to 800° C.)in the reaction chamber.

The reaction started with the flow of TiCl₄ vapor for 30-60 min and wasinterrupted by terminating the preheating of the TiCl₄ precursor andusing the bypass system, which provides continuous N₂ flow for flushingthe system. A short annealing period (10-15 min) followed, allowing thelast portions of the supplied titanium tetrachloride precursor to reactcompletely. Afterwards, the reactor was moved down for cooling. The mainportion of the synthesized material was collected on the filter. Inaddition, small portions of the product powder were found sticking tothe surfaces of the quartz reactor.

Example 2 Preparation of Fullerene-Like Nanostructures of TiS₂ in aHorizontal Reactor Based Apparatus of FIG. 2.

The reaction chamber 112 was cleaned in a similar manner as described inExample 1 above in order to maintain a water and oxygen free atmosphere.Subsequently, the reaction chamber was inserted into the oven 115.

The temperature in the horizontal reaction chamber 112 was controlled bymeans of a single-zone oven 115. The TiCl₄ vapor was prepared in theseparate evaporation chamber (14 in FIG. 1) and supplied to the reactionchamber 112 in the similar way as was done in the above-describedExample 1. The temperature of the reaction chamber 112, where the twogases (TiCl₄ and H₂S) mix and react, was varied in the range of 650-750°C. The resulting TiS₂ powder was collected for analysis on the surfaceof the reactor boat. However, the product collection was impeded as theproduct was swept by the carrier gas to the trap.

Example 3 Preparation of Fullerene-Like Nanostructures of WS₂ in aHorizontal Reactor Based Apparatus of FIG. 2.

The reaction chamber 112 was cleaned in a similar manner as described inExample 1 above in order to maintain a water and oxygen free atmosphere.

Subsequently, the reaction chamber was inserted into the oven 115.

The temperature in the horizontal reaction chamber 112 was controlled bymeans of a single-zone oven 115. The chosen precursor WO₂Cl₂ was heatedup to 270-290° C. in the separate evaporation chamber (14 in FIG. 1) andits vapor was supplied to the reaction chamber 112 in the similar way aswas done in the above-described Example 1. The temperature of thereaction chamber 112, where the two gases (metal-containing precursorand H₂S) mix and react, was varied in the range of 700-850° C. A smalloverpressure (1.1 bar) was maintained by using a gas trap filled withNaOH (5%) solution in the gas outlet of the reaction chamber.

The resulting WS₂ powder was collected for analysis on the surface ofthe reactor boat. However, the product collection was impeded as theproduct was swept by the carrier gas to the trap. The resultingnanoparticles are shown in FIG. 6. As can be noted, the IF-WS₂ obtainedin the present example are not so perfect and have hollow core. This canbe explained by the inhomegenity of the reaction parameters in thechosen horizontal reactor.

In other experiments the forming gas, containing 1-10% of H₂ in N₂, wasused instead of clean nitrogen for either caring the metal-containingprecursor or diluting the H₂S.

Furthermore, similar series of experiments were carried out usinghorizontal reactors starting with WBr₅ (boils at 333° C., preheated at290-330° C.). Different combinations of carrier gas (clear nitrogen orhydrogen-enriched nitrogen) were used. The resulting material consistedfrom IF-nanoparticles together with byproducts (platelets amorphousmaterials), as revealed by TEM analysis. Different nanoparticles bothhollow-core and non-hollow core were observed.

Example 4 Preparation of IF-WS₂ Nanoparticles in the Vertical ReactorBased Apparatus of FIG. 1

At the beginning of the process, the reaction chamber 12 was closedhermetically from outside the oven, and the reaction gases, except forprecursor (WBr₅), were supplied to the inlets flushing the system for10-15 min. The slight overpressure (ca. 1.1 bars) was maintained at aconstant value by the gas trap in the exit GO of the gases from thereaction chamber 12. Subsequently, the reactor was inserted into theoven 15.

Independent inlets IN₁ and IN₂ for both reaction gases i.e. WBr₅ and H₂Swere used, with the mixture of the reagents being formed in the reactionchamber itself. The H₂S (3-20 cc/min) was mixed with a stream of N₂ gas(10-200 cc/min in this reaction). The WBr₅ vapors were obtained bypreheating the WBr₅ precursor in the gas-washing bottle 14A of theevaporation chamber 14 and were carried to the reaction chamber 12 by anauxiliary N₂ gas flow. The temperature of the WBr₅ source was keptusually between 290 and 330° C., which is close to its boiling point of333° C. A small overpressure (1.1 bar) was maintained by using a gastrap filled with NaOH (5%) solution in the gas outlet of the reactionchamber.

The temperature in the reaction chamber zone, where the two gases (WBr₅and H₂S) mix and react, and near the filter 18 was usually variedbetween 700-850° C.

The main portion of the synthesized material was collected on thefilter. In addition, small portions of the product powder were foundsticking to the surfaces of the quartz reaction chamber.

Example 5 Preparation of IF-MoS₂ Nanoparticles in the Vertical ReactorBased Apparatus of FIG. 1

At the beginning of the process, the reaction chamber 12 was closedhermetically from outside the oven, and the reaction gases, except forprecursor (Mo(CO)₅), were supplied to the inlets flushing the system for10-15 min. Subsequently, the reaction chamber was inserted into the oven15.

Independent inlets IN₁ and IN₂ for both reaction gases i.e. Mo(CO)₅ andH₂S were used, with the mixture of the reagents being formed in thereaction chamber itself. The H₂S (3-20 cc/min) was mixed with a streamof N₂ gas (10-200 cc/min in this reaction). The Mo(CO)₅ vapors wereobtained by preheating the liquid Mo(CO)₅ in the gas-washing bottle 14Aof the evaporation chamber 14 and was carried to the reaction chamber 12by an auxiliary N₂ gas flow. The temperature of the Mo(CO)₅ source waskept usually between 160 and 200° C., which is over its melting point of150° C.

The temperature in the reaction chamber zone, where the two gases(Mo(CO)₅ and H₂S) mix and react, and near the filter 18 was usuallyvaried between 650-850° C.

The main portion of the synthesized material was collected on thefilter. In addition, small portions of the product powder were foundsticking to the surfaces of the quartz reactor.

Example 6 Preparation of IF-WS₂ Nanoparticles in the Vertical ReactorBased Apparatus of FIG. 1

At the beginning of the process, the reaction chamber 12 was closedhermetically from outside the oven, and the reaction gases, except forprecursor (WCl₄), were supplied to the inlets flushing the system for10-15 min. The slight overpressure (ca. 1.1 bars) was maintained at aconstant value by the gas trap in the exit GO of the gases from thereaction chamber 12. Subsequently, the reactor was inserted into theoven 15.

Independent inlets IN₁ and IN₂ for both reaction gases i.e. WCl₄ and H₂Swere used, with the mixture of the reagents being formed in the reactionchamber itself. The H₂S (3-20 cc/min) was mixed with a stream of N₂ gas(10-200 cc/min in this reaction). The WCl₄ vapors were obtained bypreheating the precursor in the gas-washing bottle 14A of theevaporation chamber 14 and were carried to the reaction chamber 12 by anauxiliary N₂ gas flow. The temperature of the WCl₄ source was keptusually between 200 and 400° C. in order to provide the necessary amountof precursor supplied to the reaction. A small overpressure (1.1 bar)was maintained by using a gas trap filled with NaOH (5%) solution in thegas outlet of the reaction chamber.

The temperature in the reaction chamber zone, where the two gases (WCl₄and H₂S) mix and react, and near the filter 18 was usually variedbetween 700-850° C.

The main portion of the synthesized material was collected on thefilter. In addition, small portions of the product powder were foundsticking to the surfaces of the quartz reaction chamber.

Example 7 Preparation of IF-WS₂ Nanoparticles in the Vertical ReactorBased Apparatus of FIG. 1

At the beginning of the process, the reaction chamber 12 was closedhermetically from outside the oven, and the reaction gases, except forprecursor (WCl₅), were supplied to the inlets flushing the system for10-15 min. The slight overpressure (ca. 1.1 bars) was maintained at aconstant value by the gas trap in the exit GO of the gases from thereaction chamber 12. Subsequently, the reactor was inserted into theoven 15.

Independent inlets IN₁ and IN₂ for both reaction gases i.e. WCl₅ and H₂Swere used, with the mixture of the reagents being formed in the reactionchamber itself. The H₂S (3-20 cc/min) was mixed with a stream of N₂ gas(10-200 cc/min in this reaction). The WCl₅ vapors were obtained bypreheating the WCl₅ precursor in the gas-washing bottle 14A of theevaporation chamber 14 and were carried to the reaction chamber 12 by anauxiliary N₂ gas flow. The temperature of the WCl₅ source was keptusually between 250 and 285° C. A small overpressure (1.1 bar) wasmaintained by using a gas trap filled with NaOH (5%) solution in the gasoutlet of the reaction chamber.

The temperature in the reaction chamber zone, where the two gases (WCl₅and H₂S) mix and react, and near the filter 18 was usually variedbetween 700-850° C.

Since the formal valence of tungsten in the precursor (WCl₅) differsfrom the one in the expected product (WS₂), additional reduction ofmetal was required. The excess of H₂S in the reaction atmosphere acts asthe reduction agent, however in part of the experiments additional flowof H₂ was used for this purpose. The additional flow of hydrogen (1-10%of hydrogen within nitrogen instead of pure N₂) was supplied eithertogether with precursor or mixed with H₂S.

The main portion of the synthesized material was collected on thefilter. In addition, small portions of the product powder were foundsticking to the surfaces of the quartz reaction chamber.

Example 8 Preparation of IF-WS₂ Nanoparticles in the Vertical ReactorBased Apparatus of FIG. 1

At the beginning of the process, the reaction chamber 12 was closedhermetically from outside the oven, and the reaction gases, except forprecursor (WCl₆), were supplied to the inlets flushing the system for10-15 min. The slight overpressure (ca. 1.1 bars) was maintained at aconstant value by the gas trap in the exit GO of the gases from thereaction chamber 12. Subsequently, the reactor was inserted into theoven 15.

Independent inlets IN₁ and IN₂ for both reaction gases i.e. WCl₆ and H₂Swere used, with the mixture of the reagents being formed in the reactionchamber itself. The H₂S (3-20 cc/min) was mixed with a stream of N₂ gas(10-200 cc/in in this reaction). The WCl₆ vapors were obtained bypreheating the WCl₆ precursor in the gas-washing bottle 14A of theevaporation chamber 14 and were carried to the reaction chamber 12 by anauxiliary N₂ gas flow. The temperature of the WCl₆ source was keptusually between 275 and 345° C. A small overpressure (1.1 bar) wasmaintained by using a gas trap filled with NaOH (5%) solution in the gasoutlet of the reaction chamber.

The temperature in the reaction chamber zone, where the two gases (WCl₆and H₂S) mix and react, and near the filter 18 was usually variedbetween 700-850° C.

Since the formal valence of tungsten in the precursor (WCl₆) differsfrom the one in the expected product (WS₂), additional reduction ofmetal was required. The excess of H₂S in the reaction atmosphere acts asthe reduction agent, however in part of the experiments additional flowof H₂ was used for this purpose. The additional flow of hydrogen (1-10%of hydrogen within nitrogen instead of pure N₂) was supplied eithertogether with precursor or mixed with a H₂S.

The main portion of the synthesized material was collected on thefilter. In addition, small portions of the product powder were foundsticking to the surfaces of the quartz reaction chamber.

Example 9 Preparation of IF-MoS₂ Nanoparticles in the Vertical ReactorBased Apparatus of FIG. 1

At the beginning of the process, the reaction chamber 12 was closedhermetically from outside the oven, and the reaction gases, except forprecursor (MoCl₅), were supplied to the inlets flushing the system for10-15 min. The slight overpressure (ca. 1.1 bars) was maintained at aconstant value by the gas trap in the exit GO of the gases from thereaction chamber 12. Subsequently, the reactor was inserted into theoven 15.

Independent inlets IN₁ and IN₂ for both reaction gases i.e. MoCl₅ andH₂S were used, with the mixture of the reagents being formed in thereaction chamber itself. The H₂S (3-20 cc/min) was mixed with a streamof N₂ gas (10-200 cc/min in this reaction). The MoCl₅ vapors wereobtained by preheating the precursor in the gas-washing bottle 14A ofthe evaporation chamber 14 and were carried to the reaction chamber 12by an auxiliary N₂ gas flow. The temperature of the MoCl₅ source waskept usually between 200 and 265° C. A small overpressure (1.1 bar) wasmaintained by using a gas trap filled with NaOH (5%) solution in the gasoutlet of the reaction chamber.

The temperature in the reaction chamber zone, where the two gases (MoCl₅and H₂S) mix and react, and near the filter 18 was usually variedbetween 700-850° C.

Since the formal valence of tungsten in the precursor (MoCl₅) differsfrom the one in the expected product (WS₂), additional reduction ofmetal was required. The excess of H₂S in the reaction atmosphere acts asthe reduction agent, however in part of the experiments additional flowof H₂ was used for this purpose. The additional flow of hydrogen (1-10%of hydrogen within nitrogen instead of pure N₂) was supplied eithertogether with precursor or mixed with a H₂S.

The main portion of the synthesized material was collected on thefilter. In addition, small portions of the product powder were foundsticking to the surfaces of the quartz reaction chamber.

Analysis of the Synthesized Materials

The products were analyzed mainly by means of various electronmicroscopy techniques. The following microscopes were used:environmental scanning electron microscope (Philips FEI-XL30 E-SEM);transmission electron microscope (Philips CM120 TEM), equipped with EDSdetector (EDAX-Phoenix Microanalyzer); high resolution transmissionelectron microscope (HRTEM) with field emission gun (FEI Technai F30),equipped with a parallel electron energy loss spectrometer (Gatanimaging filter-GIF (Gatan)). Simulation of the HRTEM micrographs of TiS₂was obtained using the MacTempas image-simulation software.Complementary analyses were carried out by powder X-ray diffraction(XRD).

TEM examination of the powder obtained in the horizontal set-up (Example2) revealed the presence of closed cage nanostuctures in the product(FIG. 3). The typically observed particle-size was about 100 nm, withnanoparticles ranging in size between 50 and 150 nm. The wide sizedistribution is a reflection of the inhomogenity of the reactionconditions in this set-up. The yield of the closed-cage nanoparticles inthose experiments was up to 30%, depending on the reaction conditions.The remaining material, as revealed by SEM and TEM, was made of TiS₂platelets, a few tens of nanometers to 0.5 micron in size, each.

The product of the vertical set-up (Example 1) was found to contain anappreciably larger fraction of the IF-TiS₂ phase with yields approaching80%. Furthermore, the size distribution of the synthesized nanoparticleswas found to be appreciably narrower in the vertical set-up, as comparedto the horizontal reactor. The product of the vertical reactor ended upalso to be more spherical (FIG. 4). Tilting the sample in differentviewing angles did not reveal any significant changes in the shape ofthe observed nanoparticles. These findings emphasize the advantage ofusing the vertical set-up for the synthesis of the IF-nanophasematerials. Varying the synthesis time did not seem to have anappreciable influence on the size distribution of the IF-TiS₂nanoparticles.

The resulting IF-nanoparticles were found to consist of a large numberof concentric layers displaying relatively smooth curvature. Forinstance, the nanoparticle shown in FIG. 4 consists of approximately 80concentric and spherical layers. These layers were continuous with novisible holes or edge dislocations observed. The hollow core, which wasobserved in the IF-WS₂ (MOS₂) nanoparticles, did not exist in thepresent nanoparticles. A careful examination of the synthesizednanoparticles did not reveal a spiral growth mode of the molecularlayers of the material. Instead, a quasi-epitaxial, layer by layergrowth mode could be deciphered. The observed layers are complete andare separated one from the others.

In several cases the cores of the observed TiS₂ nanoparticles were foundto be made of a number of tiny spherical IF centers, which are stackedtogether. As a rule, such nanoparticles were preferably found in theexperiments with definitely higher flow rate of TiCl₄ precursor(preheating at 130-140° C.). For instance several such centers arevisible in the TEM image of the nanoparticle shown in FIG. 3. Theborders between those nuclei can be clearly distinguished in the core ofthe nanoparticle, while the peripheral layers envelope the divided coreinto a single spherical moiety.

HRTEM image of a part of a closed TiS₂ fullerene-like nanoparticle isshown in FIG. 5A together with its simulated image. A satisfactoryagreement between the real and simulated images is indicative of thecorrect assignment of the nanoparticle's structure. It should benonetheless noted, that the simulation refers to the bulk (lT) material,which is flat, awhile the IF-TiS₂ nanoparticles are curved and theirstructure is not fully commensurate, because the number of atoms isdifferent in each of the concentric nested layers.

The interlayer distance obtained from either Fourier analysis (insert ofFIG. 4), or a direct measurement (FIG. 5B) was found to be 0.58 nm. Thisvalue represents an expansion of about 1.8% in comparison to the layerto layer separation in bulk lT-TiS₂ (0.57 nm). The interlayer distancedid not seem to vary along the entire volume of the nanoparticle. Thisresult is in a good agreement with XRD experiments, in which thesynthesized material was identified as lT-TiS₂. It nevertheless standsin a sharp contrast with the synthesized IF-WS₂ and MoS₂ nanoparticles,synthesized by reacting H₂S with the respective oxides, were often largegaps are observed between the molecular sheets. These gaps can beassociated with strain-induced brisk changes in the topology of thelayers from evenly folded to faceted structure. This topology was foundto be typical for nanoparticles which are produced by the reaction ofH₂S with the respective oxide, which starts on the surface of thenanoparticle and progresses inwards consuming the oxide core.

At high temperature experiments (800° C.), nanoparticles havingdistorted shape were observed. Also, the overall yield of the IF-TiS₂ athigh temperatures was low (app. 10%), the main portion being TiS₂platelets.

A number of other precursors were tested for their aptitude to obtainfullerene-like materials in similar way. The resulting nanoparticles ofboth MoS₂ and WS₂ (FIGS. 6-10) were obtained from variety of startingmaterials. Most of the newly-obtained nanoparticles were found to differfrom their analogs, obtained by reduction-sulfidization of oxidetemplates. More specifically, the nanoparticles obtained from the vaporsof metal-containing precursors were more spherical, with little amountof defects. Moreover, they had a small hollow core, if any, like it wasfound in the case of TiS₂.

Tribological Experiments

A ball on flat tester¹ was used for the present tribologicalexperiments. A load of 50 grams was used in these experiments. Thefriction coefficient was measured in the end of the 20 cycles run, weresteady tribological regime prevailed.

To test the efficacy of the IF-TiS₂ particles produced by the process ofthe present invention, as a solid lubricant a series of tribologicalexperiments were conducted. It was found that the addition of a smallamount (1%) of the IF-TiS₂ powder decreases significantly (10 times) thefriction coefficient of the pure oil-from 0.29 to 0.03. A similar testwith 1% bulk powder (lT-TiS₂) added to the oil, leads to a frictioncoefficient of 0.07, i.e. twice that of the IF-phase. It must beemphasized here that the portion used for the tribological testscontained no more than 50% IF-TiS₂, the rest being platelets of lT-TiS₂.The collected data suggests that the shape of the IF-TiS₂ of theinvention might play a major role in lowering the friction coefficient.The quite perfectly spherical nanoparticles with sizes ranging in the30-70 nm and up to 100 molecular layers thick obtained with the verticalset-up could provide effective rolling friction and sliding. It isemphasized the important role played by the spherical shape of thenanoparticles in providing rolling friction with a reduced frictioncoefficient and wear. These nanoparticles are also stable and compliant.

Comparison Between IF Nanoparticles Obtained in the Process of thePresent Invention and Known IF Nanoparticles:

The IF-TiS₂ nanoparticles obtained by the process of the presentinvention in a vertical reactor, typically consist of about hundredlayers and are formed fast, over a period of a few minutes or less,only. They are spherical in shape, and their lattice parameter (c) isconstant along the radial axis of the nanoparticle, which suggests thatthey suffer from relatively minor strain. Table 2 together with FIG. 11make a concise comparison between the morphology and some of theproperties of the IF-TiS₂ nanoparticles obtained by the process of thepresent invention and IF-WS₂ nanoparticles obtained by processes knownin the art.

The following Table 2 compares the representative characteristics offullerene-like WS₂ obtained by the known reaction of H₂S gas withtungsten oxide nanoparticles, and TiS₂ nanoparticles obtained fromtitanium chloride vapor according to the present invention.

TABLE 2 Comparison between representative characteristics of IF-WS₂obtained by the known reaction and IF-TiS₂ nanoparticles obtained by theprocess of the present invention. IF-TiS₂ IF-WS₂ Typical size 60-100 nm60-200 nm Number of layers 50-120 20-30 Core No core or very Emptyhollow core small core observed Overall shape of the SubstantiallyPartially faceted, nanoparticle spherical not spherical Estimated growthMinutes Hours duration Growth mechanism Nucleation and Synergetic growthsulfidization and reduction; diffusion controlled

In contrast to the earlier synthesized IF-WS₂ (MoS₂)⁵⁻⁷, the closed-cagenanoparticles of titanium disulfide produced by the process of thepresent invention have a very small hollow core or do not possess suchcore. The interlayer distance (0.58 nm) is preserved along the entirevolume of the nanoparticle. The present results are indicative of thefact that the titanium disulfide layers start to grow from a smallnuclei, obeying thereby the ubiquitous nucleation and growth mechanism.The present synthesis of IF-TiS₂ may be envisaged as a homogeneousnucleation of the fullerene-like structures from embryonic clustersformed in the vapor phase, in contrast to the heterogeneous nucleationof IF-WS₂ (MoS₂) on the surfaces of the respective oxide templates.

The vapor of TiCl₄ crosses the flux of H₂S, coming out from anoppositely placed tube at relatively high temperature (650-750° C.),which provides a high reaction rate. Since the TiS₂ clusters formed inthe gas phase are non-volatile, they condense into small nuclei. It iswell established that shrinking the size of the graphene (or otherlayered material—like TiS₂) sheet makes the planar structure unstableresulting in folding and formation of a closed-cage structure. Once suchclosed-cage nuclei of TiS₂ are formed in the vapor phase of the reactorfurther TiCl₄ adsorb on its surface and react with the H₂S gas. Thisreaction occurs in a highly controlled-quasi-epitaxial fashion, i.e.with a single growth front leading to a layer by layer growth mode. Thisgrowth mode entails minimal geometrical constraints, and hence thenanoparticles are appreciably more spherical than the previouslyreported IF nanoparticles. The spherical morphologies with relativelysmooth curvature exhibited by these nanoparticles suggest that thebending of the molecular sheets results in continuously distributeddislocations or defects, in contrast to the more facetted structures,observed in the previously synthesized IF-WS₂, where the defects arelocalized in grain boundaries. The rather large number of layersobserved in the IF-TiS₂ nanoparticles undergoing van der Waalsinteractions may compensate for the bending and dislocation energies andadd to the stability of such spherical nanoparticles.

The small crystallites, formed during the initial stages of thegas-phase reaction collide in the vapor phase. When the kinetic energyof the collision is not sufficiently large to separate the collidingnanoparticles, they aggregate forming multi-nuclei cores. Theseaggregated nanoparticles serve as a template, which are subsequentlyenfolded by additional TiS₂ layers on their surface. A fullerene-likenanoparticle with multi-core is thus obtained (see FIG. 3). The fairlynarrow size distribution of the IF-TiS₂ nanoparticles in the verticalset-up is particularly notable. Presently, two possible explanations forthis effect can be invoked. Once the nanoparticles reach a criticalsize, which coincides with their thermodynamic stability, their growthrate slows down appreciably, while the smaller nuclei continue to growfast until they reach a similar size. A further possible reason for thenarrow size distribution is that the larger nanoparticles can not floatin the vapor and they fall on the filter, where they are rapidly buriedunder the next layer of nanoparticles, and their growth slows down.

The constancy of the distance between the layers (c) in the radialdirection, and their quite perfectly spherical shape indicate that thepresent IF nanoparticles suffer little strain, only. This phenomenon isthe result of the nucleation and growth mechanism accomplished in thepresent invention, and it has a favorable impact on the tribologicalbehavior of such nanoparticles.

Other IF metal chalcogenides, e.g. IF-WS₂ and MoS₂ nanoparticles,synthesized by a similar process as the above-exemplified one for TiS₂,provide similar spherical nanoparticles consisting of many layers (FIG.6-10). It appears that the nanoparticles obtained from the vapors ofmetal-containing precursor follow the same growth mechanism (nucleationand growth). This topology favors rolling and sliding of thenanoparticles, providing improved tribological behavior for the IF solidlubricant. Since IF-WS₂ and MoS₂ are the materials of choice for suchapplications, the improved control of the nanoparticles morphology, aspresented in the present invention for IF-TiS₂, leads to a superiortribological behavior of these solid lubricants, too.

1. A process for producing inorganic fullerene-like (IF) metalchalcogenide nanoparticles, the process comprising: (a) preparing ametal precursor vapor, selected from the group consisting of metalhalide vapor, metal carbonyl vapor, organo-metallic compound vapor andmetal oxyhalide vapor, in a separate evaporation chamber; (b) feedingthe metal precursor vapor from said separate evaporation chamber into areaction chamber to flow towards a reaction zone in said reactionchamber along a vertical path to interact with at least one chalcogenmaterial in gas phase flowing towards said reaction zone in a verticalpath in a direction opposite to that of the metal precursor vapor flow,and controlling temperature conditions in said reaction zone such as toenable the formation of inorganic fullerene-like (IF) metal chalcogenidenanoparticle product.
 2. A process according to claim 1, furthercomprising: controllably varying the flow of said metal precursor vaporinto said reaction chamber to control the amount, morphology and size ofthe so-produced inorganic fullerene-like (IF) metal chalcogenidenanoparticles in solid form, the flow of the vapor of the metalprecursor being controlled by at least one of the following: (i) therate of the flow of an inert gas driving said vapor into the reactionchamber; (ii) the temperature used for heating the metal precursor toobtain a vapor of said metal precursor.
 3. The process according toclaim 1, wherein said chalcogen material contains hydrogen.
 4. Theprocess according to claim 3, wherein said chalcogen material isselected from the group consisting of H₂S, H₂Se and H₂Te and mixturesthereof.
 5. The process according to claim 1, wherein said metal isselected from the group consisting of In, Ga, Sn and a transition metalselected from the group consisting of Mo, W, V, Zr, Hf, Pt, Pd, Re, Nb,Ta, Ti, Cr and Ru.
 6. The process according to claim 1, wherein saidmetal precursor is selected from the group consisting of the followingcompounds: chromium carbonyl, chromium (III) iodide, chromium (IV)chloride, chromium (IV) fluoride, chromium (V) fluoride, chromium (VI)fluoride, cromyl chloride, trimethylgallium, hafnium bromide, hafniumchloride, hafnium iodide, trimethylindium, molybdenum carbonyl,molybdenum (V) chloride, molybdenum (V) fluoride, molybdenum (V)oxytrichloride, molybdenum (VI) fluoride, molybdenum (VI)oxytetrafluoride, molybdenum (VI) oxytetrachloride, molybdenum (VI)dioxydichloride, niobium (IV) chloride, niobium (IV) fluoride, niobium(IV) iodide, niobium (V) bromide, niobium (V) chloride, niobium (V)fluoride, niobium (V) iodide, niobium (V) oxybromide, niobium (V)oxychloride, niobium (V) dioxyfluoride, palladium (II) bromide,palladium (II) iodide, pPlatinum (II) bromide, platinum (II) chloride,platinum (II) iodide, platinum (III) bromide, platinum (III) chloride,platinum (IV) bromide, platinum (IV) chloride, platinum (IV) fluoride,platinum (IV) iodide, platinum (VI) fluoride, rhenium carbonyl, rhenium(III) bromide, rhenium (III) chloride, rhenium (III) iodide, rhenium(IV) chloride, rhenium (IV) fluoride, rhenium (V) bromide, rhenium (V)chloride, rhenium (V) fluoride, rhenium (VI) chloride, rhenium (VI)fluoride, rhenium (VI) oxytetrachloride, rhenium (VI) oxytetrafluoride,rhenium (VII) fluoride, rhenium (VII) trioxycloride, rhenium (VII)trioxyfluoride, rhenium (VII) dioxytrifluoride, rhenium (VII)oxypentafluoride, ruthenium dodecacarbonyl, ruthenium (III) bromide,ruthenium (III) chloride, ruthenium (III) fluoride, ruthenium (III)iodide, ruthenium (IV) fluoride, ruthenium (V) fluoride, tantalum (V)bromide, tantalum (V) chloride, tantalum (V) fluoride, tantalum (V)iodide, titanium (III) bromide, titanium (III) chloride, titanium (IV)bromide, titanium (IV) chloride, titanium (IV) fluoride, titanium (IV)iodide, tungsten carbonyl, tungsten (II) bromide, tungsten (II)chloride, tungsten (II) iodide, tungsten (III) bromide, tungsten (III)chloride, tungsten (V) bromide, tungsten (V) chloride, tungsten (V)fluoride, tungsten (V) oxytribromide, tungsten (V) oxytrichloride,tungsten (VI) bromide, tungsten (VI) chloride, tungsten (VI)dioxydibromide, tungsten (VI) dioxydichloride, tungsten (VI)dioxydiiodide, tungsten (VI) fluoride, tungsten (VI) oxytetrabromide,tungsten (VI) oxytetrachloride, tungsten (VI) oxytetrafluoride, vanadiumcarbonyl, vanadium (IV) chloride, vanadium (IV) fluoride, vanadium (V)fluoride, vanadyl bromide, vanadyl chloride, vanadyl dibromide, vanadyldichloride, vanadyl difluoride, vanadyl tribromide, vanadyl trichloride,vanadyl trifluoride, zirconium chloride, zirconium fluoride, andzirconium iodide.
 7. The process according to claim 1, wherein saidmetal precursor is selected from the group consisting of TiCl₄, WCl₆,WCl₅, WCl₄, WBr₅, WO₂Cl₂, WOCl₄, MoCl₅, Mo(CO)₅ and W(CO)₆, Ga(CH₃)₃,W(CH₂CH₃)₅ and In(CH₃)₃.
 8. The process according to claim 1, whereinsaid metal chalcogenide is selected from the group consisting of TiS₂,TiSe₂, TiTe₂, WS₂, WSe₂, WTe₂, MoS₂, MoSe₂, MoTe₂, SnS₂, SnSe₂, SnTe₂,RuS₂, RuSe₂, RuTe₂, GaS, GaSe, GaTe, In₂S₃, In₂Se₃, In₂Te₃, InS, InSe,Hf₂S, HfS₂, ZrS₂, VS₂, ReS₂ and NbS₂.
 9. The process according to claim1, comprising controlling the flow of the vapor of the metal precursorinto said reaction chamber by controlling the temperature used forheating the metal precursor to be in the range of between 50 degreesbelow the boiling point and up to the boiling point of said metalprecursor.
 10. The process according to claim 1, wherein the temperaturewithin the reaction zone is in the range of 500 to 900° C. 11.Fullerene-like (IF) metal chalcogenide nanoparticles obtainable by theprocess of claim
 1. 12. TiS₂ nanoparticles obtainable by the process ofclaim
 1. 13. TiS₂ nanoparticles having fullerene-like structure and adiameter size of between 10-300 nm.
 14. TiS₂ nanoparticles according toclaim 13, comprising a hollow core of a diameter substantially notexceeding 5-10 nm and of about 50-120 molecular layers.
 15. Inorganicfullerene-like (IF) MoS₂ nanoparticles obtainable by the process ofclaim
 1. 16. An apparatus for preparing IF nanostructures, the apparatuscomprising: a reaction chamber having a reaction zone, inlets forinputting reacting gases and defining a vertical path thereof towardsthe reaction zone, and an outlet; a separate evaporation chamber forseparately preparing a precursor vapor being one of the reacting gasesto be input to the reaction chamber; and a control unit configured andoperable for controlling the precursor vapor flow from the evaporationchamber into and through the reaction chamber.
 17. The apparatus ofclaim 16, wherein the reaction chamber is configured to be verticallyoriented during the apparatus operation, said gas inlets being locatedat top and bottom sides of the chamber so as to direct the precursorvapor and the other reacting gas along vertical paths in oppositedirections towards the reaction zone where the gases meet and react witheach other.
 18. The apparatus of claim 17, comprising an oven configuredto enable installation of the reaction chamber therein, the oven beingconfigured to define two temperature source regions in the reactionchamber.
 19. The apparatus of claim 16, wherein the control unitcomprises a valve arrangement associated with the evaporation chamber,said valve arrangement being configured to selectively provide a flow ofa clean inert gas substantially free of the precursor vapor. 20.Inorganic fullerene-like (IF) metal chalcogenide nanoparticles having aplurality of molecular layers and characterized in that the number ofsaid molecular layers exceeds
 40. 21. IF metal nanoparticles accordingto claim 20, having no or a very small hollow core.
 22. IF metalnanoparticles according to claim 20, comprising a hollow core of adiameter substantially not exceeding 5-10 nm and of about 50-120molecular layers.
 23. A product comprising a plurality of inorganicfullerene-like (IF) metal chalcogenide nanoparticles, a substantialportion of which having a number of molecular layers exceeding
 40. 24. Aproduct according to claim 23, wherein said substantial portion is atleast 40% out of the total number of the IF nanoparticles.
 25. Inorganicfullerene-like (IF) WS₂ nanoparticles obtainable by the process of claim1.